A global positioning system (GPS) measures the three-dimensional, global position of a radio receiver, using the distances between the radio receiver and a number of earth-orbiting satellite transmitters. The receiver, usually mounted to a vehicle such as a commercial passenger aircraft, receives signals from the satellite transmitters. Each signal indicates both the position of its transmitter and its transmission time, enabling the receiver, equipped with its own clock, to approximate signal transit times and to estimate the distances to the transmitters. A processor coupled to the receiver uses at least four of these distances, known as pseudoranges, to approximate or estimate the position of the receiver and the associated vehicle. The accuracy of these estimates, or position solutions, depends on a number of factors, for example, changing atmospheric conditions and performance of individual satellite transmitters.
In commercial aircraft navigation and guidance, global positioning systems (GPSs) have traditionally been used only for determining position of an aircraft during non-critical portions of a flight, that is, between takeoff and landing. However, in recent years, researchers have started extending GPSs for use during landings. These extended systems have taken the form of ground-augmented or differential global positioning systems which typically include two to four ground-based GPS receivers and a ground-based differential correction processor (DCP) and a correction-data transmitter, all located around an aircraft landing area.
In 1998, the FAA initiated a program to develop requirements for developing and deploying such a navigational system known as the GPS-based Local-Area-Augmentation Systems, or GPS-based LAASs. As a result of this program, the FAA released Specification, FAA-E-2937A (Apr. 17, 2002), which establishes the performance requirements for a Category I Local Ground Facility (LGF) in the LAAS system. The contents of FAA-E-2937A are incorporated herein by reference. Under this specification, the LGF will monitor the satellite constellation, provide the LAAS corrections and integrity data, and provide approach data to and interface with air traffic control.
The LAAS uses a differential global positioning system (DGPS). The DGPS includes a global positioning system (GPS) and at least one ground station. The GPS uses a number of orbiting position transmitting satellite stations and a receiver on an aircraft to determine the position of the aircraft with respect to ground. With the satellite information, the receiver can determine the position, speed, and altitude of the aircraft. By adding a ground station, the DGPS can correct errors that may occur in the transmission of data from the satellites to the receiver. As a result the DGPS can determine the position of the aircraft with a high degree of accuracy.
The ground-based GPS receivers, each with a known position, work as normal GPS receivers in determining respective sets of pseudoranges based on signals from at least four earth-orbiting satellite transmitters. These pseudoranges are fed to the ground-based DCP, which uses them and the known positions of the ground receivers to determine correction data. The correction-data transmitter then transmits to aircraft approaching the landing area. These approaching aircraft use the correction data to correct position estimates of on-board GPS receivers, providing better position solutions than possible using their on-board GPS receivers alone.
These corrected position solutions are then compared to a reference landing path to determine course deviations necessary to ensure the aircraft follows the reference landing path. The course deviations are input to an autopilot system, which guides the aircraft during automatic landings. For the autopilot system to function within safety limits set by the Federal Aviation Administration, the position estimates are required to stay within minimum accuracy limits known as vertical and lateral alert limits. Failure to stay within accuracy limits causes issuance of an alert, signaling a pilot to abort the automatic landing and to restart the landing process.
In a navigational system used in commercial aircraft, accuracy is of paramount importance. However, as in all navigational systems, a certain amount of error will inevitably exist. This error must be prepared for, monitored, and dealt with. One potential source of error identified in the LGF specification is low signal power, whether in the satellite signals received by the LGS or in the satellite and ground signals received by the aircraft. A measure of the accuracy used in navigation is the “error bound,” also referred to as the “protection limit” or “integrity limit.” The error bound reflects a range of values within which—to a predetermined confidence level set by regulations or by industry standards—the aircraft is likely to be located.